The present invention relates an isolated human Site-1 Protease promoter region. The invention also relates to screening methods for agents decreasing the expression of Site-1 protease and thereby being potentially useful for the treatment of medical conditions related to obesity and/or diabetes.
Sterol Regulatory Element-Binding Proteins (SREBPs)
The integrity of cell membranes is maintained by a balance between the amount of cholesterol and the amounts of unsaturated and saturated fatty acids in phospholipids. This balance is partly maintained by membrane-bound transcription factors called Sterol Regulatory Element-Binding Proteins (SREBPs; for reviews, see Brown and Goldstein (1997) Cell 89, 331-340; Brown and Goldstein (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 11041-11048) that activate genes encoding enzymes of cholesterol and fatty acid biosynthesis. To enhance transcription, the active NH2-terminal domains of SREBPs are released from endoplasmic reticulum membranes by two sequential cleavages. The first is catalyzed by Site-1 protease (S1P), a membrane-bound subtilisin-related serine protease that cleaves the hydrophilic loop of SREBP that projects into the endoplasmic reticulum lumen. The second cleavage, at Site-2, requires the action of S2P, a hydrophobic protein that appears to be a zinc metalloprotease. These regulated proteolytic cleavage reactions are ultimately responsible for controlling the level of cholesterol in membranes, cells, and blood.
Three isoforms of SREBPs have been identified. SREBP-1a and SREBP-1c are encoded by a single gene and differ in their N-terminal acid transcription activation domains. The N-terminus of SREBP-1a is longer and includes additional acidic amino acids, consistent with the observation that it is a more powerful transcription factor (Pai et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 40, 26138-26148). SREBP-2 is produced by a different gene and contains a long activation domain resembling that of SREBP-1a. Recent evidence suggests that the main function of SREBP-2 is to regulate cholesterol synthesis whilst that of SREBP-1 is to regulate fatty acid synthesis (Pai et al., supra).
Inhibition of SREBP transcription factor function will lead to reduced cellular synthesis of free fatty acids and cholesterol, the clinical benefits of which are expected to include increased cellular insulin sensitivity and reduced coronary artery disease (CAD). Furthermore, SREBP-1 represents a cellular mechanism for increasing both fat cell size and number (Kim et al. (1998) J. Clin. Invest. 101, 1-9). Since most obesity generally involves an increase in both cell size and cell number, inhibition of SREBP-1 might be expected to have a positive effect on obesity. The hypolipidemic effects of dietary polyunsaturated fatty acids are believed to derive from a direct inhibitory effect on SREBP-1 expression (Xu et al. (1999) J. Biol. Chem. 274, 23577-23583).
There is data indicating independent regulation of SREBP-1 and SREBP-2 in hamster liver, suggesting the possibility for specific targeting of SREBP-1 or -2 (Sheng et al. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 935-938).
Transgenic mice over-expressing a dominant-positive form of SREBP-2 in the liver and adipose tissue showed greatly increased levels of mRNAs encoding multiple enzymes of cholesterol synthesis. Enzymes involved in fatty acid synthesis were also increased, however, to a lesser extent (Horton et al. (1998) J. Clin. Invest. 101, 2331-2339). Transgenic mice over-expressing a constitutively active SREBP-1a in the liver and adipose tissue showed greatly increased mRNA levels for enzymes involved in fatty acid and cholesterol (Shimano et al. (1996) J. Clin. Invest. 98, 1575-1584). Their livers were enlarged about 4-fold due to a massive accumulation of free fatty acids and cholesterol. Over-expression of a corresponding version of SREBP-1c in adipocytes of transgenic mice yielded insulin resistance and diabetes (Shimomura et al. (1999) Genes Dev. 12, 3182-3194). In cell culture such overexpression was previously shown to promote adipocyte differentiation. It has further been shown that overnutrition increases SREBP-1c expression in liver and islets of obese fa/fa Zucker diabetic fatty rats (Kakuma, T. et al. (2000) Proc. Natl. Acad. Sci. U.S.A. 97: 8536-8541).
S-1 Protease
As discussed above, SREBPs are activated by proteolysis, which releases the active transcription factor. The luminal subtilisin-like protease Site-1 Protease (S1P) is responsible for the first of the two proteolytic steps. Cleavage by S1P enables further cleavage by a Site-2 protease. S1P is the target for feedback inhibition by cholesterol.
S1P from hamster has been cloned (Sakai et al. (1998) Molecular Cell 2, 505-514). (GenBank accession no. AF078105; SEQ ID NOS: 5 and 6). The corresponding sequence of the (then unidentified) human gene was disclosed by Nagase et al. (1995) DNA Research 2, 37-43 (GenBank Accession no. D420453; SEQ ID NOS: 3 and 4)
SREBP and S1P are co-localized with a third protein: SREBP Cleavage-Activating Protein (SCAP), which is required for Site-1 cleavage in vivo. SCAP contains a site for sterol regulation, conserved in a small number of proteins, e.g. HMG-CoA reductase.
Only one S1P has been identified among the human expressed sequence tags (ESTs). Thus, S1P may be the only member of a subfamily among the subtilisin-like proteases.
Consequently, SREBPs are important regulators of fat and sugar metabolism in mammals and direct or indirect down-regulation of SREBPs may be of therapeutic value in type II diabetes; obesity, hypercholesterolemia, and other cardiovascular diseases or dyslipidemias.
Site-1 Protease represents a molecular target for therapeutic intervention which is expected to interfere with the SREBP pathway. Two principally distinct concepts for inhibition of the site-1-protease activity may be postulated; (i) by inactivation of the proteolytic activity (classical inhibitors) or (ii) by modulation of the site-1-protease gene expression level. In order to modulate the expression of the site-1-protease gene, there is a need for identification of regulatory regions responsible for the regulation of Site-1 protease promoter. Such regulatory regions in the promoter could be used for the identification of agents that inhibit expression of Site-1 protease, and thereby for the inhibition of the SREBP pathway.
The 5xe2x80x2-flanking region (promoter region) of the human Site-1 Protease (S1P) gene has been cloned and sequenced. This promoter region is useful in biological assays for the identification of compounds that inhibit the transcription of the Site-1 Protease. Inhibition of the SREBP pathway is expected to have therapeutic value in type II diabetes; obesity, hypercholesterolemia, and other cardiovascular diseases or dyslipidemias.
Consequently, in a first aspect this invention provides an isolated human site-1 protease promoter region comprising a sequence selected from:
(a) the nucleotide sequence set forth as SEQ ID) NO: 2, or a fragment thereof exhibiting site-1 protease promoter activity;
(b) the complementary strand of (a); and
(c) nucleotide sequences capable of hybridizing, under stringent hybridization conditions, to a nucleotide sequence as defined in (a) or (b).
The term xe2x80x9cpromoter regionxe2x80x9d refers to a region of DNA that functions to control the transcription of one or more genes, and is structurally identified by the presence of a binding site for DNA-dependent RNA polymerase and of other DNA sequences on the same molecule which interact to regulate promoter function.
The nucleic acid molecules according to the present invention includes cDNA, chemically synthesized DNA, DNA isolated by PCR, genomic DNA, and combinations thereof. Genomic DNA may be obtained by screening a genomic library with the cDNA described herein, using methods that are well known in the art.
In a preferred form of the invention, the said nucleic acid molecule has a nucleotide sequence identical with SEQ ID NO: 2 of the Sequence Listing. However, the nucleic acid molecule according to the invention is not to be limited strictly to the sequence shown as SEQ ID NO: 2. Rather the invention encompasses nucleic acid molecules carrying modifications like substitutions, small deletions, insertions or inversions, which nevertheless have S1P promoter activity. Included in the invention are consequently nucleic acid molecules, the nucleotide sequence of which is at least 90% homologous, preferably at least 95% homologous, with the nucleotide sequence shown as SEQ ID NO: 2 in the Sequence Listing.
The term xe2x80x9cstringent hybridization conditionsxe2x80x9d is known in the art from standard protocols (e.g. Ausubel et al., supra) and could be understood as e.g. hybridization to filter-bound DNA in 0.5 M NaHPO4, 7% sodium dodecyl sulfate (SDS), 1 mM EDTA at +65xc2x0 C., and washing in 0.1xc3x97SSC/0.1% SDS at +68xc2x0 C.
The said xe2x80x9cfragmentxe2x80x9d (partial sequence) exhibiting site-1 protease promoter activity can be identified by the skilled person by computer-assisted sequence analysis, e.g. prediction of transcription factor binding sites.
The invention further provides a recombinant construct comprising the human site-1 protease promoter region as defined above. Preferably, the said construct comprises the S1P promoter region operably linked to a gene encoding a detectable product, in particular the human site-1 protease gene (SEQ ID NO: 3).
The term xe2x80x9clinkedxe2x80x9d indicates that a nucleotide sequence encoding a gene product and an S1P promoter, or an active fragment thereof, are located within a continuous nucleic acid sequence. The term xe2x80x9coperably linkedxe2x80x9d means that a nucleotide sequence, which can encode a gene product, is linked to the S1P promoter such that the S1P promoter regulates expression of the gene product under appropriate conditions. Two nucleotide sequences that are operably linked contain elements essential for transcription, including, for example, a TATA box.
The recombinant construct according to the invention could further comprise a reporter gene. As used herein, the term xe2x80x9creporter genexe2x80x9d means a gene encoding a gene product that can be identified using simple, inexpensive methods or reagents and that can be operably linked to a S1P promoter or an active fragment thereof. Reporter genes such as, for example, a luciferase, xcex2-galactosidase, alkaline phosphatase, or green fluorescent protein reporter gene, can be used to determine transcriptional activity in screening assays according to the invention (see, for example, Goeddel (ed.), Methods Enzymol., Vol. 185, San Diego: Academic Press, Inc. (1990); see also Sambrook, supra).
In another aspect the invention provides a vector comprising the recombinant construct as defined above. The term xe2x80x9cvectorxe2x80x9d refers to any carrier of exogenous DNA that is useful for transferring the DNA to a host cell for replication and/or appropriate expression of the exogenous DNA by the host cell. A host cell stably transformed with the recombinant construct is an additional aspect of the invention. Such a host cell can be a prokaryotic cell, a unicellular eukaryotic cell, or a cell derived from a multicellular organism. The methods employed to effect introduction of the vector into the host cell are standard methods well known to a person familiar with recombinant DNA methods. The term xe2x80x9ctransformedxe2x80x9d or xe2x80x9ctransfectedxe2x80x9d refers to the process by which exogenous DNA is transferred into an appropriate host cell.
In a further important aspect, this invention is useful in screening for pharmacological agents that modulate S1P levels by affecting the transcription of the S1P gene. As used herein, the term xe2x80x9cagentxe2x80x9d means a biological or chemical compound such as a simple or complex organic molecule, a peptide, a protein or an oligonucleotide. Consequently, this invention includes a method for identifying an agent capable of modulating the S1P promoter, comprising providing a cell comprising the S1P promoter; contacting said cell with a candidate agent; and monitoring said cell for an effect that is not present in the absence of said candidate agent.
A preferred form of the invention include a method for identification of an agent capable of decreasing or inhibiting site-1 protease promoter activity, said method comprising the steps (i) contacting a candidate agent with the human site-1 protease promoter; and (ii) determining whether said candidate agent decreases expression of the site-1 protease gene, such decrease being indicative for an agent capable of decreasing or inhibiting site-1 protease promoter activity.
For screening purposes, appropriate host cells can be transformed with a vector having a reporter gene under the control of the human S1P promoter according to this invention. The expression of the reporter gene can be measured in the presence or absence of an agent with known activity (i.e. a standard agent) or putative activity (i.e. a xe2x80x9ctest agentxe2x80x9d or xe2x80x9ccandidate agentxe2x80x9d). A change in the level of expression of the reporter gene in the presence of the test agent is compared with that effected by the standard agent. In this way, active agents are identified and their relative potency in this assay determined.
It will be understood that agents acting on the human S1P promoter can be identified by, as an additional step, analyzing direct binding interactions between the candidate agent and the human S1P promoter. Interactions with large molecules may be studied using techniques such as gel shift analysis, footprinting or NMR (see Latchman, D. S. (Ed.) (1995) Methods for studying transcription factors. In: Eukaryotic transcription factors. Academic Press, London, pp. 17-44). Small molecule compounds which appear to bind reversibly to double stranded DNA without intercalation between DNA base pairs have been defined. Methods are described by which this non-intercalative binding can be characterized using ultraviolet spectrometry, fluorimetry with ethidium as a probe, viscometry and other hydrodynamic techniques, circular dichroism and nuclear magnetic resonance spectrometry (See Baguley, B. C. (1982) Nonintercalative DNA-binding antitumour compounds. Mol Cell Biochem 43: 167-181; Gmeiner, W. H. (1998) NMR spectroscopy as a tool to investigate the structural basis of anticancer drugs. Curr Med Chem 5(2):115-135; Wemmer, D. E. and Williams, P. G. (1994) Use of nuclear magnetic resonance in probing ligand-macromolecule interactions. Methods Enzymol. 239:739-767)
A potentially useful method for identification of agents acting on the human S1P promoter is described in Swedish patent application No. 0101218-6, filed on Apr. 5, 2001. Such a method comprises the steps
(a) predicting the structure of an RNA-fragment;
(b) choosing a suitable predicted RNA-fragment of step (a), which RNA-fragment comprises at least one individual stem;
(c) synthesizing the DNA-fragment corresponding to the RNA-fragment of step (b);
(d) inserting the DNA-fragment of step (c) in the upstream proximity of a reporter assay gene, which reporter assay gene produces a signal upon translation, thereby forming a reporter construct;
(e) performing a reporter gene assay, which assay monitors the interaction between a molecule to be tested for RNA-binding and the RNA-fragment of the reporter construct.
As mentioned above, it is expected that agents capable of decreasing or inhibiting site-1 protease promoter activity have potential therapeutic value in particular in obesity, and in type II diabetes; hypercholesterolemia, atherosclerosis and other cardiovascular diseases or dyslipidemias. Consequently, the invention comprises a method for the treatment of medical conditions related to obesity, comprising administering to a patient in need thereof a therapeutically effective amount of an agent identified by the method according to the invention.
The term xe2x80x9ctreatmentxe2x80x9d means any treatment of a diseases in a mammal, including: (i) preventing the disease, i.e. causing the clinical symptoms of the disease not to develop; (ii) inhibiting the disease, i.e. arresting the development of clinical symptoms; and/or (iii) relieving the disease, i.e. causing the regression of clinical symptoms. The term xe2x80x9ceffective amountxe2x80x9d means a dosage sufficient to provide treatment for the disease state being treated. This will vary depending on the patient, the disease and the treatment being effected.
Throughout this description the terms xe2x80x9cstandard protocolsxe2x80x9d and xe2x80x9cstandard proceduresxe2x80x9d, when used in the context of molecular biology techniques, are to be understood as protocols and procedures found in an ordinary laboratory manual such as: Current Protocols in Molecular Biology, editors F. Ausubel et al., John Wiley and Sons, Inc. 1994, or Sambrook, J., Fritsch, E. F. and Maniatis, T., Molecular Cloning: A laboratory manual, 2nd Ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. 1989.